Astronomy & Astrophysics manuscript no. paper116 c ESO 2020 September 3, 2020 Dozens of virtual impactor orbits eliminated by the EURONEAR VIMP DECam data mining project O. Vaduvescu1, 2, 3, L. Curelaru4, M. Popescu5, 2, B. Danila6, 7, and D. Ciobanu8 1 Isaac Newton Group of Telescopes, Apto. 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain 2 Instituto de Astrofisica de Canarias (IAC), C/Via Lactea s/n, 38205, La Laguna, Tenerife, Spain 3 University of Craiova, Str. A. I. Cuza nr. 13, 200585, Craiova, Romania 4 EURONEAR member, Brasov, Romania 5 Astronomical Institute of the Romanian Academy, 5 Cutitul de Argint, 040557, Bucharest, Romania 6 Astronomical Observatory Cluj-Napoca, Romanian Academy, 15 Ciresilor Street, 400487 Cluj-Napoca, Romania 7 Department of Physics, Babes-Bolyai University, Kogalniceanu Street, 400084 Cluj-Napoca, Romania 8 EURONEAR collaborator, Bucharest, Romania Submitted 15 June 2020; Reviewed 23 July 2020; Accepted 27 July 2020 ABSTRACT Context. Massive data mining of image archives observed with large etendue facilities represents a great opportunity for orbital amelioration of poorly known virtual impactor asteroids (VIs). There are more than 1000 VIs known today; most of them have very short observed arcs and many are considered lost as they became extremely faint soon after discovery. Aims. We aim to improve the orbits of VIs and eliminate their status by data mining the existing image archives. Methods. Within the European Near Earth Asteroids Research (EURONEAR) project, we developed the Virtual Impactor search using Mega-Precovery (VIMP) software, which is endowed with a very effective (fast and accurate) algorithm to predict apparitions of candidate pairs for subsequent guided human search. Considering a simple geometric model, the VIMP algorithm searches for any possible intersection in space and time between the positional uncertainty of any VI and the bounding sky projection of any image archive. Results. We applied VIMP to mine the data of 451,914 Blanco/DECam images observed between 12 September 2012 and 11 July 2019, identifying 212 VIs that possibly fall into 1286 candidate images leading to either precovery or recovery events. Following a careful search of candidate images, we recovered and measured 54 VIs in 183 DECam images. About 4,000 impact orbits were eliminated from both lists, 27 VIs were removed from at least one list, while 14 objects were eliminated from both lists. The faintest detections were around V∼24.0, while the majority fall between 21<V<23. The minimal orbital intersection distances remains con- stant for 67% detections, increasing for eight objects and decreasing for ten objects. Most eliminated VIs (70%) had short initial arcs of less than five days. Some unexpected photometric discovery has emerged regarding the rotation period of 2018 DB, based on the close inspection of longer trailed VIs and the measurement of their fluxes along the trails. Conclusions. Large etendue imaging archives represent great assets to search for serendipitous encounters of faint asteroids and VIs. Key words. Virtual impactor asteroids; Data mining software; Blanco telescope; DECam image archive 1. Introduction a few teams of students and amateur astronomers who searched the entire Canada-France-Hawaii Legacy Survey (CFHTLS) Exploiting the huge potential data wealth of the astronomical MegaCam archive (Vaduvescuetal. 2011), the European image archives has been one of the main astrometric aims of Southern Observatory Max Planck Telescope (ESO/MPG) the European Near Earth Asteroids Research (EURONEAR) 1 Wide Field Imager (WFI) and the Isaac Newton Telescope project , besides new observations of fainter near Earth asteroids (INT) Wide Field Camera (WFC) archives (Vaduvescu et al. (NEAs), which typically require 2-4 m class telescopes that arXiv:2009.00807v1 [astro-ph.IM] 2 Sep 2020 2013) and the Subaru Suprime-Cam archive (Vaduvescu et al. are difficult to allocate for routine observations. Particularly, 2017). Together, we improved the orbits of 408 NEAs based on potential hazardous asteroids (PHAs), virtual impactors (VIs), 425,000 images contained in the respective archives. and imminent impactors are in most need of such data for orbital amelioration, which can improve and characterise their orbits Our second project, Mega-Precovery, and few related tools and improve our knowledge of potential future impacts with our have gathered 112 instrument archives in the Mega-Archive planet. Structured Query Language (SQL) database, a meta-data collection holding more than 16 million images able to search Our first such data mining project, Precovery, presented the immediately for one given known or unknown moving (Solar avenues for the search for NEAs and PHAs in a single image System) object or any fixed (stellar or extra-galactic) object archive (Vaduvescu et al. 2009). This tool was applied later by (Vaduvescu et al. 2020). This big data mining resource is simi- 2 Send offprint requests to: O. Vaduvescu lar to the Canadian Solar System Object Image Search (SSOIS) e-mail: [email protected] 1 www.euronear.org 2 http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/ssois/. Article number, page 1 of 18 A&A proofs: manuscript no. paper116 (Gwyn et al. 2012) (for moving objects) and TELARCHIVE3 of VIMP software in 2015-2016 and 2018, respectively, testing or other online observatory tools to provide searches of major them with the Canada-France-Hawaii Telescope (CFHT) Mega- archive collections (only for fixed objects). cam, then the Blanco DECam archive, whose first findings were announced by Vaduvescu et al. (2019a). Little similar data mining work have been conducted by other authors during recent years. Solano et al. (2014) present The paper is organised as follow. In Sect. 2 we intro- a citizen-science project implemented online at the Spanish duce the VI databases and derive some statistics, presenting Virtual Observatory (SVO) that aims to improve the orbits of the VIMP algorithm and software. In Sect. 3 we introduce the NEAs serendipitously falling into the Sloan Digital Sky Survey Blanco/DECam archive, presenting some statistics and applying (SDSS) Data Release 8 (DR8) survey (938,046 images). This it to the VIMP project and other four large etendue archives. We project engaged 3,226 registered users who identified known present the DECam results in Sect. 4, followed by conclusions NEAs and measured their positions, improving the orbital and future projects in Sect. 5. elements of 551 NEAs (including only 29 extended arcs). Besides astrometric data mining projects, physical char- 2. Virtual impactor data mining acterisation of NEAs, main belt asteroids (MBAs), and other First, we define virtual impactor asteroids and remind the two ex- orbital classes can be derived based on major imaging surveys. isting virtual impactor databases. Second, we present the VIMP Popescu et al. (2016) conducted the Moving Objects from algorithm and software. VISTA survey (MOVIS) project for the physical characterisa- tion of moving objects recovered from the Visible and Infrared Survey Telescope for Astronomy (VISTA) VHS-DR3 survey 2.1. Virtual Impactors (at the time comprising 86,562 stack images in four filters). The modern statistical grounds concerning the recovery and The authors derived near-infrared magnitudes and colours of pairing of poorly observed asteroids, their multiple orbital 39,947 objects (including 38,428 MBAs and 52 NEAs, among solutions, close planetary encounters, and potential risk of other classes). Again using the MOVIS database, Popescu et al. collision were paved two decades ago by the celestial me- (2018) provided taxonomic classifications of 6,496 asteroids, chanics group of Andrea Milani at the University of Pisa reporting the albedo distribution for each taxonomic group and (Milani 1999; Milani and Valsechi 1999; Milani et al. 2000; new median values for the main types. Milani and Gronchi 2009). Briefly, the region of uncertainty defined by any asteroid is characterised by a set of multiple Mahlke et al. (2018) present a method to acquire positions, solution orbits that acceptably fit the available observations, be- photometry, and proper motion measurements of Solar System sides the nominal orbit solution, which fits the observations best objects in surveys using dithered image sequences. The authors (Milani et al. 2005). A virtual asteroid represents a hypothetical mined the data from a fraction of 346 sq. deg of the sky imaged minor planet that follows any orbit included in the region of in up to four filters by the VST/OmegaCAM Kilo-Degree survey uncertainty (Milani et al. 2005). A virtual impactor (VI) is (KiDS), reporting 20,221 candidate asteroids (including 46.6% defined as a virtual asteroid whose orbit could impact Earth or unknown objects). another planet sometime in the future (although with a very small probability), according to current observations (Milani Cortes-Contreras et al. (2019) present two complementary 2005). methods to identify asteroids serendipitously observed in astronomical surveys. They applied these methods to the 6.4 sq. deg of the sky (30,558 images in the J-band) covered by the United Kingdom Infrared Telescope (UKIRT) Wide Field 2.2. Virtual impactor databases Camera (WFCAM) Transit Survey, identifying 15,661 positions More than 22,600 NEAs are known today (April 2020)4 thanks of 1,821 asteroids (including 182 potential new discoveries and to the major U.S. lead surveys, from which about one thousand a few NEA candidates). are VIs, and this number